The mission is designed to vastly increase the discovery space of the high energy transient phenomena over the entirety of cosmic history. Its primary scientific goals will address the Early Universe ESA Cosmic Vision themes “How did the Universe originate and what is made of?” (4.1, 4.2 and 4.3) and will also impact on “The gravitational wave Universe” (3.2) and “The hot and energetic Universe” themes. This is achieved via a unique payload providing an unprecedented combination of: 1) wide and deep sky monitoring in a broad energy band (0.3keV - 20 MeV); 2) focusing capabilities in the soft X-ray band providing large grasp and high angular resolution; and 3) on board near-IR capabilities for immediate transient identification and redshift determination.

The foreseen payload of THESEUS includes the following instrumentation:

The mission profile includes: an onboard data handling units (DHUs) system capable of detecting, identifying and localizing likely transients in the SXI and XGIS FOV; the capability of promptly (within a few tens of seconds at most) transmitting to ground the trigger time and position of GRBs (and other transients of interest); and a spacecraft slewing capability of ~10-20°/min). The baseline launcher / orbit configuration is a launch with Vega-C to a low inclination low Earth orbit (LEO, ~600 km, <5°), which has the unique advantages of granting a low and stable background level in the high-energy instruments, allowing the exploitation of the Earth’s magnetic field for spacecraft fast slewing and facilitating the prompt transmission of transient triggers and positions to the ground.

A sketch of the THESEUS satellite showing the instruments accommodation is shown on the left panel. The middle panel shows the cumulative distribution of GRBs with redshift determination as a function of the redshift for Swift (in 10 yr) and the prediction for THESEUS (in 3 yr). The panel on the right shows the sensitivity of THESEUS for different classes of sources.

The main scientific goals of the proposed mission are to:

Explore the Early Universe (cosmic dawn and reionization era) by unveiling a complete census of the Gamma-Ray Burst (GRB) population in the first billion years. Specifically to:

Perform unprecedented studies of the global star formation history of the Universe up to z ~ 10 and possibly beyond;

Detect and study the primordial (pop III) star population: when did the first stars form and how did the earliest pop III and pop II stars influence their environments?

Investigate the re-ionization epoch, the interstellar medium (ISM) and the intergalactic medium (IGM) up to z ~ 8 - 10: how did re-ionization proceed as a function of environment, and was radiation from massive stars its primary driver? How did cosmic chemical evolution proceed as a function of time and environment?

Investigate the properties of the early galaxies and determine their star formation properties in the re-ionization era.

Perform an unprecedented deep monitoring of the X-ray transient Universe in order to:

Locate and identify the electromagnetic counterparts to sources of gravitational radiation and neutrinos, which may be routinely detected in the late ‘20s / early ‘30s by next generation facilities like aLIGO/aVirgo, eLISA, ET, or Km3NET;

Provide real-time triggers and accurate (~1 arcmin within a few seconds; ~1’’ within a few minutes) locations of (long/short) GRBs and high-energy transients for follow-up with next-generation optical-NIR (E-ELT, JWST if still operating), radio (SKA), X-rays (ATHENA), TeV (CTA) telescopes;

Provide unprecedented insights into the physics and progenitors of GRBs and their connection with peculiar core-collapse SNe and substantially increase the detection rate and characterization of sub-energetic GRBs and X-Ray Flashes;

Fill the present gap in the discovery space of new classes of high-energy transient events, thus providing unexpected phenomena and discoveries.

By satisfying the requirements coming from the above main science drivers, the THESEUS payload will also automatically enable excellent observatory science opportunities, including, e.g., performing IR observatory science, especially providing capability for response to external triggers, thus allowing strong community involvement. We remark that THESEUS has survey capabilities for high-energy transient phenomena complementary to the Large Synoptic Survey Telescope (LSST) in the optical. Their joint availability in the next decade would enable a remarkable scientific synergy between them.

Left: THESEUS will have the ideal combination of instrumentation and mission profile for detecting all types of GRBs (long, short/hard, weak/soft, high-redshift), localizing them from a few arcmindown to arsecand measure the redshift for a large fraction of them. Middle: expected X-ray fluxes at peak luminosity for z=0.05 and from different models of magnetar-powered X-ray emission from long-lived NS-NS merger remnants. Predictions from each model are represented by a coloured region and/or by single dots that are indicative of fiducial cases (see the legend on the right). Grey solid lines show typical GRB X-ray afterglows observed with Swift/XRT. The dashed/solid curves show the SXI sensitivity vs. exposure time, assuming a source column density of 5x1020 cm−2 (i.e., well out of the Galactic plane and very little intrinsic absorption, solid line) and 1022 cm−2 (significant intrinsic absorption, dashed line). Right: the detection, study and arcsecond localization of afterglow and kilonova emission from short GRB/GW events will be possible with THESEUS/IRT. In this figure, we show in particular the light curve of the kilonova associated to the gravitational wave/short GRB event GW170817/GRB170817A in the IRT filters. The continuous and dashed red lines indicate the THESEUS/IRT limiting H magnitudes for imaging and prism spectroscopy, respectively, with 300s of exposure.

THESEUS synergies in the field of the early Universe exploration (left) and the time-domain multi-messenger Astrophysics (right).